Hello Commander Spiral 0ut!
In general, the ability to fly back and pick up a ship has a few issues: that you are doing the journey twice (even if the first journey is in a faster ship) and that you are forced to leave a ship at the pick up location. In addition, your time is completely filled by the logistics task. The ability to have ships delivered, even at non-optimal jump rates, is arguably better.
Secondly, just because we currently don't have bulk freighters in the game does not mean that we won't have things like this in the future (regardless of whether we have timed or instant ship deliveries) or even that they don't exist in the lore already (I believe the concept of mega freighters has always existed in Elite lore in some way).
And although we may finesse the final times to be a little faster or slower, the concept is that these freighters are built to be able to get everywhere, with minimal crew, but not amazingly quickly.